Just A Little More Volume Please...

[unclej]

i just picked up a little class A amp pretty cheap because it hummed so bad and was "touch sensitive". after installing a three prong plug and moving the electrolytics from where they had been shoved in on top of everything the hum is gone. it's a little four watt amp with a nice jenson speaker upgrade and sounds wonderful. but...i'd like to see if there's anything that can be done to add just a bit of volume to it. at present it has a 12au6, a 50c5 for the power tube and a 35w5 rectifier tube. can any of the tubes be changed to give it just a little more boost? if so which one and to what and what will it do to the bias?

[Paul Marossy]

I don't know about the tubes, but it's amazing how that ground connection on the power cord helps with the hum, isn't it?

[unclej]

it really is pretty amazing. and i can't prove it but i'm guessing that straightening out the rat's nest of wiring that whoever worked on it last left had to have helped a little. even if it didn't it appeals to my sense of order and universal balance....

[lovekraft]

Hey, does that thing have a power transformer winding for the heaters? Some of those used a 50 volt winding or a bias tap on the transformer secondary, but some of them ran the heaters in series with a resistor directly from the AC line, and that's a suicide-friendly scenario! Just check it out, please - don't want to lose anybody.

[Paul Marossy]

I'm sure that messin' with the wiring probably helped, too. I found that out in a big way when I built my Matchless Spitfire clone! I had a horrible hum that moving one wire about 1/8" completely eliminated.

And as lovecraft says, be careful!

[unclej]

when i grow up i want to be just like you lk. you were right. i did a little research and found an article on some of the old japanese wiring systems for both amps and am radios and it seems that my little amp uses one of their basic techniques for calculating heater voltages and adjusting heater currents without using a power transformer. in my case it's a 150mA series. one leg of the power cord is wired directly to the #1 pin of the rectifier tube then through a resistor to the heater and through another to the plate and so on down the line.

the article also says that using a three wire cord and grounding the chassis makes it safe. what do you think?

[lovekraft]

Well, the guys over at AX84 call them "Widowmakers" or "3-Tube Deathtraps"! It was (probably) safe enough in old radios, because the knobs were insulated (usually Bakelite), but the guitar amp input will hook up your axe directly to the mains, so if there's a problem, well,... the consensus seems to be that you probably won't die (immediately), but it won't be pleasant.

Using a grounded plug and grounding the chassis is still putting your life in the hands of the electrician who wired the outlet. The very least I would do is install a 120v:120v isolation transformer -as it is, if something fails, and you become the ground path, you're hooked directly to the AC line, so you can pull current until the fuse blows. I have issues enough with high voltage, I damn sure don't want to deal with line current! Whatever you do, don't ever play it barefoot, or standing in a puddle! Just be careful!!

[unclej]

thanks as always lk...what i find interesting about this little amp is that it's another alamo..supposedly made in san antonio but they evidently either bought them from japan or borrowed their methods to save a little money. if they're really that dangerous i'll just keep this one here at the shop as part of my collection. it's not really any more than a practice amp anyway.